A Spectroscopically Confirmed Excess of 24 micron Sources in a Super Galaxy Group at z=0.37: Enhanced Dusty Star Formation Relative to the Cluster and Field Environment

Abstract

To trace how dust-obscured star formation varies with environment, we compare the fraction of 24 micron sources in a super galaxy group to the field and a rich galaxy cluster at z~0.35. We draw on multi-wavelength observations that combine Hubble, Chandra, and Spitzer imaging with extensive optical spectroscopy (>1800 redshifts) to isolate galaxies in each environment and thus ensure a uniform analysis. We focus on the four galaxy groups in supergroup 1120-12 that will merge to form a galaxy cluster comparable in mass to Coma. We find that 1) the fraction of supergroup galaxies with SFR(IR)>3 Msun/yr is four times higher than in the cluster (32% vs. 7%); 2) the supergroup's infrared luminosity function confirms that it has a higher density of IR members compared to the cluster and includes bright IR sources not found in galaxy clusters at z<0.35; and 3) there is a strong trend of decreasing IR fraction with increasing galaxy density, i.e. an IR-density relation, not observed in the cluster. These dramatic differences are surprising because the early-type fraction in the supergroup is already as high as in clusters, i.e. the timescales for morphological transformation cannot be strongly coupled to when the star formation is completely quenched. The supergroup has a significant fraction (~17%) of luminous, low-mass, IR members that are outside the group cores (R>0.5 Mpc); once their star formation is quenched, most will evolve into faint red galaxies. Our analysis indicates that the supergroup's 24 micron population also differs from that in the field: 1) despite the supergroup having twice the fraction of E/S0s as the field, the fraction of IR galaxies is comparable in both environments, and 2) the supergroup's IR luminosity function has a higher L(IR)* than that previously measured for the field.